Recycled aluminium silicate material and a particulate mixture comprising recycled aluminium silicate material

ABSTRACT

A recycled aluminium silicate material, suitable for use in ceramic article production, wherein the recycled aluminium silicate material has a particle size distribution such that: (i) the d 50  particle size is from 10 μm to 30 μm; (ii) the d 70  particle size is less than 40 μm; and (iii) the d 98  particle size is less than 60 μm. A particulate mixture, suitable for use in ceramic article production, includes the above defined recycled aluminium silicate material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase Application filed under 35U.S.C. § 371 claiming benefit to International Patent Application No.PCT/EP2019/059867, filed Apr. 16, 2019, which claims priority toEuropean Application Nos. EP18167919.2, EP18167910.1 and EP18167937.4,each filed Apr. 18, 2018, and each of which applications are herebyincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a recycled aluminium silicate material,and to a particulate mixture comprising a recycled aluminium silicatematerial. The particulate mixture and recycled aluminium silicatematerial are suitable for use in ceramic article production, andespecially ceramic porcelain floor tiles.

BACKGROUND OF THE INVENTION

There is continuing interest in the use of recycled materials inparticulate mixtures that can be used in ceramic production, such asrecycled aluminium silicate material. Recycled aluminium silicatematerial is typically derived from coal combustion products, such as flyash. This has environmental benefits.

However, the use of such recycled aluminium silicate material in themanufacture of ceramic articles, especially ceramic porcelain floortiles, and especially when used at higher levels, can lead to problems,especially with the glaze of the glazed ceramic articles. This isparticularly relevant for large scale production processes.

The use of recycled aluminium silicate material brings greatervariability into the production process compared to conventional ceramiccompositions. This greater variability is more significant when higherlevels of recycled aluminium silicate material are used.

The ceramic articles, and especially ceramic porcelain floor tiles, canshrink in an inconsistent manner and/or excessive, which in turn canlead to internal stresses. This can cause defects such as fractures orweak internal structures that are prone to fracture, for example uponhandling.

Other problems include black coring, and poor water absorptionproperties. This is especially important for ceramic porcelain floortiles.

The present invention seeks to provide a recycled aluminium silicatematerial and a particulate mixture that can be used to make ceramicarticles, and especially ceramic porcelain floor tiles, that compriserecycled aluminium silicate material, and that exhibit no black coring,limited shrinkage, and has excellent water absorption (i.e. low)properties.

The present invention controls the particle size distribution, and inparticular the d₅₀ particle size, to provide these good properties ofavoiding black coring, having optimal shrinkage and low waterabsorption. Using a particle size that is too small results in unwantedblack coring and excessive shrinkage, whilst increasing the particlesize too much results in poor (high) water absorption properties due tolack of vitrification. These result in very poor-quality ceramicarticles such as porcelain floor tiles. Using a particle sizedistribution, and especially the d₅₀ particle size, as required by thepresent invention overcomes these problems and provides for a ceramicarticle having no black coring, acceptable shrinkage, and very low waterabsorption properties. This is highly desirable for ceramic porcelainfloor tiles and their production. Ceramic porcelain floor tiles need tobe strong and impermeable to water, and in addition they areparticularly susceptible to problems of black coring. For these reasonsthe particulate mixture and recycled aluminium silicate material of thepresent invention, are especially suitable for use in ceramic porcelainfloor tile production, and especially ceramic grès porcelain floortiles.

The present invention provides a particulate mixture and recycledaluminium silicate material that enables the incorporation of recycledaluminium silicate material into ceramic articles, such as ceramicporcelain floor tiles, having excellent properties.

SUMMARY OF THE INVENTION

The present invention relates to a recycled aluminium silicate material,suitable for use in ceramic article production, wherein the recycledaluminium silicate material has a particle size distribution such that:(i) the d₅₀ particle size is from 10 μm to 30 μm; (ii) the d₇₀ particlesize is less than 40 μm; and (iii) the d₉₈ particle size is less than 60μm. The present invention also relates to a particulate mixture,suitable for use in ceramic article production, comprising the abovedefined recycled aluminium silicate material.

DETAILED DESCRIPTION OF THE INVENTION

Particulate Mixture:

Typically, the particulate mixture is suitable for use in ceramicarticle production, and especially suitable for use in ceramic porcelainfloor tile production. Typically, a ceramic production process involvesdelivering the particulate mixture to a press, such as a hydraulicpress, and pressing the particulate mixture into a green article. Thegreen article is then transferred to a dryer, then to a kiln, andsintered or fired to form a ceramic article. Standard ceramic productionprocesses known in the art are suitable.

The particulate mixture typically comprises from 30 wt % to 80 wt %recycled aluminium silicate material, preferably from greater than 50 wt% to 80 wt %, or from 60 wt % to 80 wt %, or even from 70 wt % to 80 wt% recycled aluminium silicate material. The particulate mixture may alsocomprise from 30 wt % to 75 wt %, or from 30 wt % to 70 wt %, or fromabove 35 wt % to 75 wt %, or from above 35 wt % to 70 wt %, or from 40wt % to 75 wt %, or from 40 wT % to 70 wt % recycled aluminium silicatematerial. The recycled aluminium silicate material is described in moredetail below.

The particulate mixture comprises from 20% to 70 wt %, or from 20 wt %to 50 wt % or less, or rom 20 wt % to 40 wt %, or from 20 wt % to 30 wt%, or from 30 wt % to 60 wt % material selected from clay, shale,feldspar, glass and any combination thereof. A preferred material is acombination of clay and feldspar. A suitable clay is a standard claysuch as Ukrainian clay. A preferred clay is a combination of standardclay and high plasticity clay. The weight ratio of standard clay to highplasticity clay may in the range of from 2:1 to 5:1. Suitable clay is ahigh plasticity clay such as bentonite clay. Typically, high plasticityclay has an Attterburg's plasticity index of greater than 25.0.Typically, standard clay has an Atterburg's plasticity index of 25.0 orless.

Typically, the particulate mixture has a particle size distribution suchthat:

-   -   (i) the d₅₀ particle size is from 10 μm to 30 μm;    -   (ii) the d₇₀ particle size is less than 50 μm; and    -   (iii) the d₉₈ particle size is less than 70 μm.

Preferably, the particulate mixture has a particle size distributionsuch that:

-   -   (i) the d₅₀ particle size is from 10 μm to 25 μm;    -   (ii) the d₇₀ particle size is less than 30 μm; and    -   (iii) the d₉₈ particle size is less than 55 μm

The d₅₀ particle size of the particulate mixture is typically in therange of from 10 μm to 30 μm, or from 10 μm to 25 μm, or from 15 μm to20 μm. The d₇₀ particle size of the particulate mixture is typicallyless than 50 μm, or less than 40 μm, or less than 30 μm, and typicallyis in the range of from 15 μm to 35 μm, or from greater than 20 μm to 30μm. The d₉₈ particle size of the particulate mixture is typically lessthan 75 μm, or less than 70 μm, or less than 60 μm, or less than 55 μm,and typically is in the range of from 40 μm to less than 60 μm, or from45 μm to less than 55 μm.

The particulate mixture may also have a particle size distribution suchthat the d₁₀ particle size is in the range of from 3 μm to 12 μm, orfrom 4 μm to 11 μm. The particulate mixture may also have a particlesize distribution such that the d₉₀ particle size is less than 50 μm, orless than 45 μm, or less than 40 μm, or is in the range of from 30 μm to40 μm. The particulate mixture may also have a particle sizedistribution such that the d₃₀ particle size is in the range of fromabove 6 μm to 20 μm, or from above 10 μm to 15 μm.

The particle size distribution of the particulate mixture can becontrolled by any combination of milling, classification and/orblending. Separation of particulate mixtures into a coarse fraction (orcut) and a fine fraction (or cut) is conveniently done by airclassification when there are smaller particles which would blind thescreens used in mechanical sieves. The size of the coarse and finefractions can be determined by the operation of the classifier. Atypical example is the Micron Separator Air Classifier from HosokawaMicron. The machinery is able to classify particles by balancing thecentrifugal force of the rotor and the centripetal force of the air.Material to be separated is pulled through by the fan into the inletduct and up to the rotor, where the two opposing forces classify it.Finer particles are more susceptible to centripetal forces whereascoarse particles are more prone to centrifugal force. These forces flowcoarse materials down the inside wall of the machine, emptying out thematerials in the coarse particle discharge, while finer particles travelthrough the air current into the rotor and then discharged through theupper outlet duct. By changing the rotational speed of the internalrotor, the size of the coarse and fine cuts can be easily adjusted.Increasing the speed of the rotor will increase the size of the splitbetween the coarse and fine fractions.

Comminution systems very commonly include a mill in combination with aclassifier. These classifiers can be closely integrated with the mill,for example the Hosokawa Micron Air Classifier Mill MS 1500 AC, orseparate pieces of equipment. Suitable mills include the Palla VibratingMill from MBE Coal and Mineral GmbH or the Hosokawa Micron MikroPulverizer® Hammer & Screen Mill. The particle distribution can beadjusted by the selective blending of different sized fractions so as tochange the whole distribution of the mix rather than just an upper or alower size limit.

The particle size distribution of the particulate mixture can becontrolled by any suitable particle size control techniques. Onesuitable technique is air classification.

The particle size distribution of the particulate mixture is carefullycontrolled in the present invention. Standard ceramic mixtures known inthe art and routinely used in ceramic production processes, have largerparticle size distributions, for example having a d₉₈ particle size of63 μm or larger. By controlling the particle size distribution of theparticulate mixture, the present invention provides a particulatemixture that is suitable for use in ceramic production, and whichresults in a ceramic article having a much smoother surface. Withoutwishing to be bound by theory, the inventor believes that the particlesize distribution of the particulate mixture required by the presentinvention ensures that any impurity that may be present in theparticulate mixture and that may end up on the surface of any resultantceramic article is not significantly noticeable by the human eye. Inaddition, the particle size distribution required by the presentinvention also ensures that any combustible impurities present in theparticulate mixture can ignite during the heating stage of the ceramicproduction. This ensures that these impurities are burnt out before thesintering stage of the ceramic production process, typically before thepores of the article close, which helps avoid black coring. This allowsany combustion gases to escape from the article, which in turn protectsthe article from any unwanted bloating during the ceramic productionprocess.

The particulate mixture may comprise a binder, typically from 0.1 wt %to 3.0 wt % binder, or from 0.5 wt % to 2.0 wt % binder. Suitablebinders are described in more detail below. Typically, the incorporationof binder into the particulate mixture imparts sufficient strength tothe resultant green article which is formed from the particulatemixture, for example by pressing, during a ceramic production process.

The particulate mixture can be made by blending the individualcomponents in any mixer capable of imparting sufficient shear to the mixto disperse the materials. Suitable mixers could include high shearmixers such as the Hosokawa Micron Flexomixer series or Loedige CBmixers. Other suitable mixer could include lower shear mixers such asribbon blenders or paddle mixers or continuous screw mixers. Suitablemixers include the Model 50×10 Ribbon Blender from Morton Mixers Ltd andthe Bella Model XN paddle mixer from Dynamic Air. The mixers can becontinuous or batch. An alternative is to use pneumatic mixing where theindividual materials are continuously fed to and conveyed by a pneumaticconveying system. The materials typically become highly dispersed, andwell mixed in the air stream within the pneumatic conveying system.Other suitable types of continuous mixer are conveying screws,especially those with back-mixing elements to enhance mixing. An exampleof a suitable conveying screw mixer for making the particulate mix isthe MESC200 model from Hydroscrew Ltd. High-shear mixers are capable ofmaking the particulate mixture from the separate materials by mixing,typically for a few seconds. Lower shear mixers will typically take from1 to 10 minutes of mixing to make suitable homogenous mixtures. Anotherpossible way to make the particulate mixture is to feed the individualmaterials into a grinder. The high shear and intense air flows andintense mixing typically experienced in a mill will very rapidly createa homogenous mixture which can then be classified as needed. Most millswill be very capable of mixing the materials as well as grinding them,including any mills described earlier.

The recycled aluminium silicate can be mixed with other components suchas clays and/or feldspars using the mixers and methods describedearlier. The various materials can be added separately to a mixer or canbe pre-mixed in process equipment prior to the main mixing.

A preferred process for making the particulate mixture incorporatingrecycled aluminium silicate is to use feed the different materials intoa comminution process and use any milling step, in combination withpneumatic transport and blending, to blend and mix the individualmaterials so as to form a homogenous mixture.

Other materials may need to be added to the particulate mixture, such aspolymers and plasticisers, as needed to include specific properties orto increase the green strength of the ceramic article prior to firing.Any materials which are in powder form can be added as above. However,any material in aqueous solution or suspension will typically need to beadded in a high-shear mixer such as the Flexomixer or similar, or theLoedige CB. Any liquid materials added to the mix will typically need tobe finely dispersed into the mix by using a high-shear mixing step tofinely disperse the liquid throughout the particulate mix.

Any other material that can be added to impart specific properties tothe finished ceramic article can be incorporated into the particulatemixture during the blending process.

Preferably, the particulate mixture is not prepared by a spray-drying.Such unsuitable spray-drying process steps involved making an aqueousslurry of the recycled aluminium silicate material and any other solidmaterial, such as the clay and/or feldspar, and spray-drying the aqueousslurry to form the particulate mixture. Such processes involve asignificant amount of energy and are not desirable.

The particulate mixture may comprise from 0.5 wt % to 8.0 wt %, or from1.0 wt % to 8.0 wt %, or from 1.0 wt % to 7.0 wt %, or from 1.0 wt % to6.0 wt %, or from 1.0 wt % to 5.0 wt %, or from 1.0 wt % to 4.0 wt %, orfrom 1.0 wt % to 3.0 wt % combustible carbon. The recycled aluminiumsilicate material may comprise greater than 2.0 wt % to 8.0 wt %, orfrom 2.5 wt % to 7.0 wt % combustible carbon.

The particulate mixture may comprise from 0.5 wt % to 12.0 wt %, or from0.5 wt % to 11.0 wt %, or from 0.5 wt % to 10 wt %, or from 0.5 wt % to9.0 wt %, or from 0.5 wt % to 8.0 wt %, or from 0.5 wt % to 7.0 wt %, orfrom 0.5 wt % to 6.0 wt %, or from 0.5 wt % to 5.0 wt %, or from 0.5 wt% to 4.0 wt %, or from 0.5 wt % to 3.0 wt %, or from 0.5 wt % to 2.0 wt% iron oxide.

Recycled Aluminium Silicate Material:

Typically, the recycled aluminium silicate material is derived from coalcombustion products.

Typically, the recycled aluminium silicate material is obtained bysubjecting the coal combustion products, such as ash, to a beneficiationprocess. The recycled aluminium silicate is typically beneficiated flyash.

Typically, the recycled aluminium silicate material is obtained bysubjecting the coal combustion products, such as ash, to an initialparticle size screen (such as a 1 mm screen) to remove any largeobjects, and then to one or more smaller particle size screens (such as250 μm and/or 125 μm) to remove large particles. This screened materialis then typically subjected to a magnetic separation step to reduce theiron oxide content. This magnetic separation step can involve a firstmagnetic separation step, for example at a gauss of 8,000 or around8,000, followed by a second magnetic separation step, for example at agauss of 30,000, or around 30,000. Alternatively, only one magneticseparation step may be used, for example at a gauss of 8,000 or around8,000. This material is then typically subjected to a carbon reductionstep, such as calcination or flotation, preferably calcination. Thematerial may also be subjected to an electrostatic separation technique.

The recycled aluminium silicate material is typically predominatelyaluminium silicate. The recycled aluminium silicate material typicallycomprises combustible carbon and iron oxide; and may additionallycomprise trace amounts of other materials such as sodium salts and/ormagnesium salts, and metal oxides other than iron oxide, such as sodiumoxide, potassium oxide and titanium oxide. The recycled aluminiumsilicate material typically comprises at least 88 wt % aluminiumsilicate, preferably at least 90 wt % aluminium silicate. Depending onthe levels of the combustible carbon and iron oxide, the recycledaluminium silicate may even comprise at least 92 wt %, or at least 94 wt%, or at least 96 wt %, or even at least 98 wt % aluminium silicate.

The recycled aluminium silicate material has a particle sizedistribution wherein:

-   -   (i) the d₅₀ particle size is from 10 μm to 30 μm;    -   (ii) the d₇₀ particle size is less than 40 μm; and    -   (iii) the d₉₈ particle size is less than 60 μm.

The recycled aluminium silicate material may have a particle sizedistribution wherein:

-   -   (i) the d₅₀ particle size is from 10 μm to 25 μm;    -   (ii) the d₇₀ particle size is less than 30 μm; and    -   (iii) the d₉₈ particle size is less than 55 μm.

The recycled aluminium silicate material typically has a particle sizedistribution such that at least 99 wt % of the recycled material has aparticle size of below 75 micrometers. The recycled aluminium silicatematerial may have a particle size distribution such that substantiallyall of the recycled material has a particle size of below 75micrometers.

The recycled aluminium silicate material may also have a particle sizedistribution such that the d₁₀ particle size is in the range of from 3μm to 10 μm, or from 4 μm to 6 μm. The recycled aluminium silicatematerial may also have a particle size distribution such that the d₃₀particle size is in the range of from above 6 μm to 20 μm, or from above10 μm to 15 μm. The recycled aluminium silicate material may also have aparticle size distribution such that the d₉₀ particle size is less than50 μm, or less than 45 μm, or less than 40 μm, or is in the range offrom 30 μm to 40 μm.

The particle size distribution of the recycled aluminium silicatematerial can be controlled by similar means as described above forcontrolling the particle size distribution of the particulate mixture.The particle size distribution of the recycled aluminium silicatematerial can be controlled by any combination of milling, classificationand/or blending.

The recycled aluminium silicate material may comprise:

-   -   (a) optionally, from 1.0% to 5.0% combustible carbon; and    -   (b) optionally, from 0.5% to 3.0% iron oxide.

The recycled aluminium silicate material may comprise from 0.5 wt % to8.0 wt %, or from 1.0 wt % to 8.0 wt %, or from 1.0 wt % to 7.0 wt %, orfrom 1.0 wt % to 6.0 wt %, or from 1.0 wt % to 5.0 wt %, or from 1.0 wt% to 4.0 wt %, or from 1.0 wt % to 3.0 wt % combustible carbon. Therecycled aluminium silicate material may comprise greater than 2.0 wt %to 8.0 wt %, or from 2.5 wt % to 7.0 wt % combustible carbon.

One preferred recycled aluminium silicate material is obtained byremoving all of the combustible carbon from the coal combustion product,and then adding combustible carbon back to this nil-combustible carbonmaterial. In this way, the level of combustible carbon present in therecycled aluminium silicate material can be carefully, and tightly,controlled.

The level of combustible carbon present in the recycled aluminiumsilicate material can be controlled, typically reduced, by techniquessuch as calcination, electrostatic removal, and flotation techniquessuch as froth-air flotation techniques.

Such processes for controlling the level of combustible carbon are welldescribed in the art.

Suitable equipment for calcining materials to reduce carbon levelsinclude the Staged Turbulent Air Reactors supplied by SEFA Group ofLexington, S.C. These reactors heat incoming ash to further burn out theresidual carbon.

Another well used technique is triboelectrostatic separation wherebycarbon particles can be removed from the bulk ash material, especiallyafter comminution, by passing through an electrostatic separator. Thecarbon particles can be charged to have an opposite charge to thenon-carbon particles and can then be removed by passing the ash materialthrough an electrostatic separator. Suitable equipment for this includethe STET separators supplied by ST Equipment and Technologies LLC ofNeedham, Mass.

Suitable froth flotation equipment includes the Dorr-Oliver and Wemcounits supplied by FLSmidth.

These processes can all reduce excessively high carbon levels. Incalcination processes, increasing the operating temperatures willfurther reduce the carbon levels. In electrostatic separation,increasing the voltages used in the separation units, and increasing thedegree of comminution of the material entering the separator, can beused to further reduce the carbon levels.

In froth flotation processes, increasing the degree of milling of theincoming material to further release unburnt carbon particles,increasing the amount of air used and using additives such assurfactants, can all be used to control the reduction in the levels ofcarbon.

Carbon levels can be increased by the addition of finely-groundcombustible carbon-rich materials into the particulate mixture. It maybe especially preferable to add any combustible carbon-rich materialinto any comminution steps involved in the preparation of theparticulate mixture. It is also preferred if the combustible carbon-richmaterial is that material previously extracted from combustiblecarbon-rich ash. This maximises efficiency. Other sources, such asground coal, can certainly be used. Preferably, the particle size of thecombustible carbon-rich material in the particulate mixture iscomparable to the particle sizes of the other materials present in theparticulate mixture.

The recycled aluminium silicate material may comprise from 0.5 wt % to12.0 wt %, or from 0.5 wt % to 11.0 wt %, or from 0.5 wt % to 10 wt %,or from 0.5 wt % to 9.0 wt %, or from 0.5 wt % to 8.0 wt %, or from 0.5wt % to 7.0 wt %, or from 0.5 wt % to 6.0 wt %, or from 0.5 wt % to 5.0wt %, or from 0.5 wt % to 4.0 wt %, or from 0.5 wt % to 3.0 wt %, orfrom 0.5 wt % to 2.0 wt % iron oxide.

One preferred recycled aluminium silicate material is obtained byremoving all of the iron oxide from the coal combustion product, andthen adding iron oxide back to this nil-iron oxide material. In thisway, the level of iron oxide present in the recycled aluminium silicatematerial can be carefully, and tightly, controlled.

The iron oxide level in the recycled aluminium silicate is typicallycontrolled by a process of detecting the iron oxide level in theparticulate mixture and, if it is out of spec, then either increasingthe amount of iron oxide removed from the recycled aluminium silicate oradding iron-oxide rich material into the aluminium silicate.

Iron oxide levels can be reduced by passing the recycled aluminiumsilicate through one or more magnetic separators. These apply a magneticfield to the passing stream of recycled aluminium silicate which allowsmagnetically-susceptible materials, such as iron oxide, to be removedfrom the bulk stream. Magnetic materials such as magnetite can beremoved by using a lower intensity magnetic field of up to 10,000 Gauss(=1 Tesla). Less magnetically susceptible minerals such as hematite canalso be extracted using magnetic separation but typically need a muchhigh magnetic intensity field of up to 2 or 3 Tesla. Often magneticseparation processes will use a low intensity separation step followedby a high intensity separation step. Suitable equipment for extractionof iron oxide from recycled aluminium silicate includes the WDY range ofmagnetic separators made by the Foshan Wandaye Machinery EquipmentCompany Ltd of Foshan City, Guangdong, China. The model WD-7A-300 couldbe used. Magnetic separation could also be done on wet slurries but thisis not a preferred route for treating recycled aluminium silicate due tothe need for a secondary drying step.

The iron oxide level in the recycled aluminium silicate can be increasedby the controlled addition of iron oxide rich material to the recycledaluminium silicate. Iron oxide minerals such as magnetite or hematitewould be most preferable but other sources could be used. An especiallypreferred solution would be the re-utilisation of iron oxides removedfrom prior processing of recycled aluminium silicate with excessivelyhigh levels of iron oxide. Preferably, the iron oxide rich particleshave a comparable size to the recycled aluminium silicate so as toensure homogeneity. The iron-oxide rich material could be added to therecycled aluminium silicate prior to any mixing or milling steps to aidhomogeneity.

Preferably, the recycled aluminium silicate material comprises:

-   -   (a) from 1.0% to 3.0% combustible carbon; and    -   (b) from 0.5% to 2.0% iron oxide.

Combustible Carbon:

Typically, combustible carbon is carbon that can be measured by a losson ignition (LOI) method. It is this combustible carbon that needs to becarefully controlled in the particulate mixture. The recycled aluminiumsilicate material may comprise non-combustible carbon such asnon-combustible carbide, typically at very low levels (trace amounts).

Iron Oxide:

Typically, the iron oxide content is measured by x-ray fluorescencespectrometry (XRF).

Binder:

Suitable optional binders are organic binders. Suitable organic bindersinclude polyvinyl alcohol, superplasticizers, methylcellulose,carbomethoxy cellulose, or dextrin. Other binders will be known to thoseskilled in the art. The organic binder may be in the form of a liquid.

Method of Measuring the Combustible Carbon Content:

The level of combustible carbon is measured by the Loss on Ignition(LOI) test as per ASTM D7348. In this test, 1 g of fly ash is firstdried at 150° C. to dry the sample. The sample is then cooled weighed.Then the sample is heated in a step wise manner over a two-hour periodto reach 500° C.

Method of Measuring Iron Oxide Content:

The level of iron oxide is measured by X-ray fluorescence. The typicalparticle size of the recycled aluminium silicate is sufficiently smallthat the technique is suitable for accurate measurement. The techniqueworks by the excitation of the sample using high energy gamma or X-rays.This causes an ionisation of the atoms present which then emitcharacteristic frequency EM radiation which is dependent on the type ofatom. Analysis of the intensity of different frequencies allows anelemental analysis to be made. Suitable equipment would be the Vartarange of XRF analyzers supplied by Olympus. The equipment detectselemental iron and the result is most usually converted to thecorresponding level of Fe₂O₃.

Method of Measuring Particle Size Distribution:

The particle size distribution is measured by laser diffraction. Asuitable standard for size analysis by laser diffraction is given in ISO8130-13 “Coating powders—Part 13. Particle size analysis by laserdiffraction”. Suitable analysers meeting this standard are made byHoriba Instruments of Irvine, Calif., USA; Malvern Instruments ofWorcestershire, United Kingdom; Sympatec GmbH of Clausthal-Zellerfeld,Germany; Beckman-Coulter of Fullerton, Calif., USA. A suitable sizeanalyser is the Mastersizer 2000 by Malvern Instruments. Typically, the“dry” analysis technique is used where the material is tested as apowder stream, rather than the wet method where the test material isdispersed in a fluid first.

The measurement is typically done as per the manufacturer's instructionmanual and test procedures.

The results are typically expressed in accordance with ISO 9276-1:1998,“Representation of results of particle size analysis—Part 1: GraphicalRepresentation”, Figure A.4, “Cumulative distribution Q3 plotted ongraph paper with a logarithmic abscissa”.

EXAMPLES

Three ceramic compositions, containing equal amounts of recycledaluminium silicate material, clay and feldspar, but with differentparticle size distributions, were prepared and formed into green ceramictest articles of equal dimensions and weights.

These green test articles were then all fired simultaneously in an oven,thus experiencing identical conditions. The resulting ceramic articleswere tested for evidence of black coring and for water absorption. Thetesting showed that only the composition with the inventive particlesize distribution made acceptable ceramics without black coring orunacceptably high moisture absorption.

Three samples of recycled aluminium silicate material containing 3.0 wt% Fe₂O₃ and 7.6 wt % carbon were mixed with clay and sodium feldspar tomake a mixture having the following properties. Particle sizedistributions and iron oxide and carbon levels of the recycled aluminiumsilicate test materials are shown below.

Recycled Recycled Recycled aluminium silicate aluminium silicatealuminium silicate material A material B material C d50 (μm) 7.4 21.340.8 d70 (μm) 9.8 28.1 49.2 d99 (μm) 22.2 53.0 54.9 Fe₂O₃ (wt %) 3.0 3.03.0 Carbon (wt %) 7.6 7.6 7.6

Each of the recycled aluminium silicate materials was then mixed withthe indicated amounts of clay and feldspar to make three test batches ofthe following compositions and properties.

Test Batch A Test Batch B Test Batch C (comparative) (invention)(comparative) Recycled 60.8 60.8 60.8 aluminium silicate material (wt %)Clay (wt %) 23.4 23.4 23.4 Feldspar (wt %) 9.4 9.4 9.4 Fe₂O₃ (wt %) 1.81.8 1.8 Carbon (wt %) 4.6 4.6 4.6 d50 (μm) 6.6 20.1 38.0 d70 (μm) 9.1 2645.8 d99 (μm) 26.2 48.6 50.9

The three compositions were then used to make ceramic test articles. Tengrams of each mixture were mixed with 0.8 g of 10 wt % aqueous dextrinsolution, and each wetted mixture was pressed into a test ceramic diskof diameter 26 mm and thickness 10 mm using a pressure of 40 MPa. Thesample disks were dried at 110° C. to constant weight (about 4 hours),then fired together in an oven at a constant ramp of 3° C./min to atemperature of 1230° C., soaked for 15 minutes at the top temperature,and then cooled for 7 hours.

After this, the samples were then visually inspected for evidence ofblack coring, measured for their water absorption according to ISO10545-3, and radial shrinkage expressed as percent reduction in theinitial diameter of the test ceramic disk. These values provide anindicator for the degree of vitrification of the ceramic articles.

Results are as follows:

Test Batch A Test Batch B Test Batch C (comparative) (invention)(comparative) Black coring Present Not present Not present Waterabsorption 0.2% 1.2% 8.6% Radial  11% 8.4%   5% Shrinkage

The data shows the benefits of the inventive particle size distributionfor producing porcelain tiles with high levels of recycled materials,and with sufficient clay and other materials to make industrially robustceramic articles.

The invention claimed is:
 1. A particulate mixture, suitable for use inceramic article production, wherein the mixture comprises recycledaluminium silicate material, wherein the recycled aluminium silicatematerial has the following particle size distribution: (i) a d₅₀particle size is from 10 μm to 30 μm; (ii) a d₇₀ particle size is lessthan 40 μm; and (iii) a d₉₈ particle size is less than 60 μm, asmeasured by laser diffraction as a dry dispersion.
 2. A particulatemixture according to claim 1, wherein the particulate mixture comprisesfrom 30 wt % to 80 wt % recycled aluminium silicate material.
 3. Aparticulate mixture according to claim 1, wherein the particulatemixture comprises from 40 wt % to 70 wt % recycled aluminium silicatematerial.
 4. A particulate mixture according to claim 1, wherein theparticulate mixture has the following particle size distribution: (i)the d₅₀ particle size of from 10 μm to 25 μm; (ii) the d₇₀ particle sizeof less than 30 μm; and (iii) the d₉₈ particle size of less than 55 μm.5. A particulate mixture according to claim 1, wherein the particulatemixture comprises from 30 wt % to 60 wt % material selected from clay,shale, feldspar, glass or any combination thereof.
 6. A particulatemixture according to claim 1, wherein the recycled aluminium silicatematerial has a particle size distribution such that at least 99 wt % ofthe recycled aluminium silicate material has a particle size of below 75micrometers.
 7. A particulate mixture according to claim 1, wherein therecycled aluminium silicate comprises: a) from 0.5% to 8.0% combustiblecarbon; and b) from 0.5% to 12.0% iron oxide.
 8. A particulate mixtureaccording to claim 1, wherein the recycled aluminium silicate comprises:a) from 1.0% to 5.0% combustible carbon; and b) from 0.5% to 3.0% ironoxide.
 9. A particulate mixture, suitable for use in ceramic articleproduction, wherein the mixture comprises from 40 wt % to 70 wt %recycled aluminium silicate material, wherein the recycled aluminiumsilicate material has the following particle size distribution: (i) ad₅₀ particle size is from 10 μm to 30 μm; (ii) a d₇₀ particle size isless than 40 μm; and (iii) a d₉₈ particle size is less than 60 μm,wherein the particulate mixture comprises from 30 wt % to 60 wt %material selected from clay, shale, feldspar, glass or any combinationthereof.